1. Field
This embodiment relates to a demodulation circuit and an electronic device, and more particularly, to a demodulation circuit for demodulating multilevel signals received on radio and an electronic device for receiving on radio and demodulating multilevel signals.
2. Description of Related Art
Non-contact data carriers including non-contact IC cards and non-contact ID chips, which do not contain power sources such as batteries, obtain electric power from radio wave energy emitted from reader/writer devices, and at the same time perform data communication by using the radio waves. Therefore, the non-contact data carriers and the reader/writer devices have modulation circuits for modulating data and demodulation circuits for demodulating data.
The demodulation circuits installed in the non-contact data carriers and the reader/writer devices calculate from a received signal a threshold to be used for determining whether the received signal is H level or L level. Based on the threshold, the demodulation circuits obtain prescribed received data from the received signal.
FIG. 10 is a view showing circuit blocks of a demodulation circuit to be used in a non-contact data carrier. As shown in this figure, the demodulation circuit has a wave detector 101, a maximum/minimum detector 102, an intermediate value calculator 103, and a comparator 104.
The wave detector 101 detects a radio signal RF received by an antenna. The radio signal RF received by the antenna has been subjected to ASK modulation, for example. By getting the radio signal RF through a lowpass filter or the like, an envelope that is a basis of received data D0 is obtained. The maximum/minimum detector 102 detects the maximum value and the minimum value of the signal received from the wave detector 101. The intermediate value calculator 103 calculates a threshold to be used for determining whether the received radio signal RF is H level or L level, from the maximum value and the minimum value detected by the maximum/minimum detector 102. For example, a threshold is calculated by adding the maximum value and the minimum value and dividing the result by two. The comparator 104 detects L state and H state depending on whether the signal received from the wave detector 101 is greater than the threshold calculated by the intermediate value calculator 103. Then, the received data D0 of the detected L state and H state is output to a circuit of latter stage.
It should be noted that there exists an automatic threshold value control circuit for always playing digital signals completely, irrespective of amplitude fluctuation and fluctuation of direct current level of signals. (For example, refer to JP 10-261940 A).